I'm going to guess that you're referring to an immersion chiller with a whirlpool arm. In order to take advantage of the fact that you're whirlpooling, you'll need to stop the pump and let the wort settle for a few minutes so that the trub/hop/debris cone can form. Once this has happened, you will then want to drain out of the bottom drain on your kettle. Since you need to be careful not to disturb the cone, gravity is probably the best approach, but I suppose you could use the pump if you wanted to. Draining through the whirlpool arm would defeat the purpose of whirlpooling. Hope that helps.